Mechanisms to facilitate random access by link-budget-limited devices

ABSTRACT

Mechanisms enabling link-budget-limited (LBL) devices to more effectively perform random access may include: (1) broadcasting a Physical Random Access Channel (PRACH) configuration index (PCI) reserved for LBL devices; (2) configuring LBL devices to use a PCI that is offset from the conventional PCI of current cell; (3) configuring LBL devices to transmit PRACH messages using an alternative set of subframes, different from conventionally-defined subframe set; (4) configuring LBL devices to transmit PRACH messages on odd frames when the conventional PRACH configuration specifies even frames; (5) configuring LBL devices to generate and use extra PRACH preambles that are not used by non-LBL devices; (6) configuring LBL devices to use group B preambles while non-LBL devices are configured to use group A preambles; and (7) boosting power of a random access response message after an Nth random access failure with preamble conforming to an LBL-reserved pattern of preambles.

PRIORITY CLAIM INFORMATION

This application claims priority to:

-   -   U.S. Provisional Application No. 62/197,698, filed Jul. 28,        2015, titled “Random Access Mechanisms for Link-Budget-Limited        Devices”, by Li Su, Sami M. Almalfouh, and Venkateswara Rao        Manepalli; and    -   U.S. Provisional Application No. 62/193,657, filed on Jul. 17,        2015, titled “Paging Link Budget Limited User Devices”, by Li        Su, Sami M. Almalfouh, Srinivas Burugupalli, Srinivasan Nimmala,        Venkateswara Rao Manepalli, and Vijay Kumar Ramamurthi.        All of above identified Applications are incorporated by        reference in their entireties as though fully and completely set        forth herein.

FIELD

The present application relates to wireless communication, and moreparticularly, to mechanisms capable of enhancing random access procedurefor user equipment devices that are link budget limited.

DESCRIPTION OF THE RELATED ART

Wireless user equipment (UE) devices such as smart phones and tabletcomputers communicate with wireless networks to perform any of a widevariety of functions such as telephone calls, Internet browsing, email,text messaging, social media updates, navigation using the globalpositioning system (GPS), etc.

In LTE, the random access procedure (referred to herein as “RACH”) is aprocedure for synchronizing the user equipment (UE) device with thenetwork (NW). RACH may be used for: initial access by the UE device tothe NW; handover of the UE device from one cell to another; RRCre-establishment; uplink and/or downlink data arrival; positioning inRRC connected. RACH is a procedure to allow the UE to access the NW, tosynchronize with uplink signals from different UE devices, and to obtainorthogonal resources. Thus, it is important to ensure that the messagesof the RACH are successfully communicated.

Some wireless devices may however be link budget limited (LBL), andthus, experience difficulties in receiving messages transmitted by basestations of the network. The base stations may likewise experiencedifficulties in receiving messages transmitted by thelink-budget-limited devices. A device may be link budget limited for anyof various reasons, e.g., if

-   -   the antenna system of the device is performing poorly; or    -   the antenna system of the device is designed to fit within a        housing too small for optimum transmission and/or reception        performance in the bands of interest;    -   the device is located far from the base station; or    -   obstructions intervene between the base station and the device        (e.g., if the device is located inside a building); or    -   battery power of the device is limited.

A link-budget-limited UE device may have the limited RF range indownlink (receive) direction and/or in the uplink (transmit) direction.Thus, there exists a need for mechanisms capable of enhancing theability of link-budget-limited devices and base stations to efficientlyexchange messages. In particular, there exists a need for mechanismscapable of improving the performance of the random access procedure forlink-budget-limited UE devices. It would be desirable if such mechanismscould be compatible with (and/or easily extendible from) the existingLTE network, e.g., minimum or no impact on LTE network capacity, and/or,minimum or no impact on LTE physical layers, to facilitate ease ofimplementation.

SUMMARY

In one set of embodiments, the base station may be configured tobroadcast an alternative PRACH configuration index for use bylink-budget-limited devices, where the alternative PRACH configurationindex is different from a conventional PRACH configuration index of thecell. The link-budget-limited devices may be configured to transmitPRACH messages using the PRACH configuration identified by thealternative index while non-LBL devices (and/or legacy devices) use thePRACH configuration identified by the conventional index. (Thealternative PRACH configuration index may be selected so that thecorresponding PRACH configuration has an allowable set of temporalopportunities for PRACH transmission that is disjoint from the allowableset of the conventional PRACH configuration.) The base station receivesPRACH messages from the LBL devices based on the alternative PRACHconfiguration, and receives PRACH messages from the non-LBL devices(and/or legacy devices) based on the conventional PRACH configuration.Thus, when the base station receives a given PRACH message, the basestation may easily determine whether or not the PRACH message wastransmitted by an LBL device. The alternative index may be signaled bythe base station in a new system information block, e.g., a new systeminformation block created for the purpose of signaling RACH parameter(s)to LBL devices.

In one set of embodiments, LBL devices may apply an offset to theconventional PRACH configuration index of the cell, to obtain anLBL-specific index. The LBL devices may transmit PRACH messages usingthe PRACH configuration identified by the LBL-specific index whilenon-LBL devices (and/or legacy devices) use the PRACH configurationidentified by the conventional index. (The offset may be determined sothat the PRACH configuration corresponding to the LBL-specific index hasan allowable set of temporal opportunities for PRACH transmission thatis disjoint from the allowable set of the conventional PRACHconfiguration.)

In one set of embodiments, LBL devices may transmit PRACH messages usinga modified PRACH configuration that has (a) the same PRACH format andallowable set of frames (SFNs) as the conventional PRACH configurationof the cell, and (b) an allowable set of subframes that is disjoint fromthe allowable subframe set of the conventional PRACH configuration. (Inthe context of LTE, the conventional PRACH configuration of the cell isidentified by the PRACH configuration index signaled in the systeminformation block of type2.) Non-LBL devices (and/or legacy devices) mayuse the conventional PRACH configuration to transmit their PRACHmessages.

In one set of embodiments, the LBL devices may transmit PRACH messagesin odd frames when the conventional PRACH configuration of the cellspecifies the use of even frames. Non-LBL devices (and/or legacydevices) may respect the conventional PRACH configuration, including itsrestriction to even frames, when transmitting PRACH messages. The basestation can thus easily determine whether or not a given PRACH messagecorresponds to an LBL device based on whether the PRACH message occursin an odd-numbered frame or an even-numbered frame.

In one set of embodiments, an LBL device may be configured to generatean expanded set of preambles, including extra preambles beyond aconventional set of preambles for the cell. The LBL device may select(e.g., randomly select) a preamble from the extra preambles, instead offrom the conventional set. The selected preamble may then be used forthe transmission of a PRACH message. The base station can thus easilydetermine whether or not a given PRACH message corresponds to an LBLdevice based on whether the included preamble is one of the extrapreambles or a conventional preamble.

In one set of embodiments, a conventional set of PRACH preambles (e.g.,the conventional set defined by an existing wireless communicationstandard such as LTE) may be partitioned into two groups, i.e., group Aand group B. The base station may configure the use of groups A and B sothat non-LBL devices (and/or legacy devices) will select preambles fromgroup A and LBL devices will select preambles from group B. Thus, whenthe base station receives a given PRACH message, the base station mayeasily determine whether or not the PRACH message was transmitted by anLBL device by determining the group membership (A or B) of the preambleincluded in the PRACH message.

The base station may employ any of the above-described mechanisms todetermine whether a given PRACH message was transmitted by an LBLdevice. In response to determining that the PRACH message wastransmitted by an LBL device, the base station may employ any of variousmechanisms to enhance the likelihood of successful completion of therandom access procedure. For example, the base station may boost thetransmission power of the random access response (msg2) transmitted tothe LBL device in response to the received PRACH message, i.e., powerboost relative to the power that would be used for a non-LBL device. (Inthe context of LTE, the random access response may be transmitted withan RA-RNTI consistent with the RA-RNTI of the received PRACH message.)As another example, the base station may employ a more complex decodingalgorithm and/or receive beamforming to increase the likelihood ofsuccessfully decoding msg3 of the random access procedure. As yetanother example, the base station may boost the power of downlinktransmissions to the UE device after the random access procedure iscompleted. As yet another example, the base station may instruct the UEdevice to employ lower coding rate (more redundancy) and/or lowermodulation order for uplink transmissions.

In one set of embodiments, the base station may count the number offailed random access attempts whose preambles are consistent with anLBL-specific pattern of preamble indices or preamble index offsets. Whenthe count exceeds a given threshold, the base station may start boostingthe power of msg2 (i.e., the random access response message) on anyfollowing random access attempt whose preamble is consistent with theLBL-specific pattern.

The status of a device as being link budget limited may be a permanentcondition or a variable condition. For example, some devices may be linkbudget limited when located far from a serving base station, but becomenon-link-budget-limited (non-LBL) when located closer to the basestation. Some devices may be link budget limited by design, e.g., byvirtue of having an antenna system of small size or having limited powerto expend due to smaller battery capacity, etc.

If a UE device is link budget limited (LBL) by design, the UE device mayperform any of the presently disclosed methods without an explicit stepof determining if the UE device is link budget limited (or has beenclassified as being link budget limited). For example, wherever one ofthe presently disclosed methods refers to a UE device as performing anaction (or set of actions) “in response to a determination that the UEdevice is link budget limited”, a UE device that is LBL by design mayperform that action (that set of actions) without a step of determiningLBL status. Knowledge of LBL status may be built into the softwareand/or hardware that controls the UE device.

Note that the techniques described herein may be implemented in and/orused with a number of different types of devices, including but notlimited to, base stations, access points, cell phones, portable mediaplayers, tablet computers, wearable devices, and various other computingdevices.

This Summary is intended to provide a brief overview of some of thesubject matter described in this document. Accordingly, it will beappreciated that the above-described features are merely examples andshould not be construed to narrow the scope or spirit of the subjectmatter described herein in any way. Other features, aspects, andadvantages of the subject matter described herein will become apparentfrom the following Detailed Description, Figures, and Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present subject matter can be obtainedwhen the following detailed description of the embodiments is consideredin conjunction with the following drawings.

FIG. 1A illustrates an example of a wireless communication systemaccording to some embodiments.

FIG. 1B illustrates an example of the base station 102 in communicationwith three wireless devices 106 ₁, 106 ₂ and 106 ₃, according to someembodiments.

FIG. 2 illustrates a base station 102 in wireless communication withwireless devices 106A and 106B, according to some embodiments.

FIG. 3 illustrates an example of a wireless communication systemaccording to some embodiments.

FIG. 4 illustrates an example of a base station according to someembodiments.

FIG. 5A illustrates a PRACH preamble being transmitted as part of anuplink frame. (PRACH is an acronym for Physical Random Access Channel.)

FIG. 5B illustrates the structure of a conventional PRACH, according toone possible format.

FIG. 6 illustrates a cyclic prefix (CP) and sequence portion of a PRACH.

FIG. 7 illustrates messages that may be exchanged between a userequipment (UE) device and base station (e.g., eNodeB) as part of arandom access procedure.

FIG. 8 is a copy of the PRACH configurations listed in Table 5.7.1-2 of3GPP TS 36.211.

FIG. 9 illustrates a method according to some embodiments, enabling alink-budget-limited UE device to transmit a PRACH message using analternative PRACH configuration index, different from theconventionally-signaled PRACH configuration index.

FIG. 10 illustrates a method according to some embodiments, enabling alink-budget-limited UE device to transmit a PRACH message using a PRACHconfiguration index that is offset from the conventionally-signaledPRACH configuration index.

FIGS. 11 and 12 illustrate methods according to some embodiments,enabling a link-budget-limited UE device to transmit a PRACH messageusing an allowable set of subframes different from that identified bythe conventionally-signaled PRACH configuration index.

FIG. 13 illustrates a method according to some embodiments, enabling alink-budget-limited UE device to transmit a PRACH message using an oddframe when the conventional PRACH configuration specifies even frames asthe allowable set of PRACH transmission frames.

FIG. 14 illustrates a method according to some embodiments, enabling alink-budget-limited UE device to generate additional preambles beyond aconventionally-defined set of preambles, and to transmit a PRACH messageusing a selected one of the additional preambles.

FIG. 15 illustrates a method according to some embodiments, enabling alink-budget-limited UE device to perform PRACH transmission using apreamble selected from one subset of a conventionally defined set ofpreambles while non-LBL device use another subset of the conventionallydefined set.

FIG. 16A illustrates a method according to some embodiments, enabling aUE device to signal its status as a link-budget-limited device (to abase station) by using a predetermined sequence of PRACH preamble indexoffsets for successive PRACH transmissions, until a current PRACHtransmission results in success of the random access procedure.

FIG. 16B illustrates a method according to some embodiments, enabling aUE device to signal its status as a link-budget-limited device (to abase station) by using a sequence of PRACH preamble index offsets.

FIG. 17 illustrates a method according to some embodiments, enabling abase station to facilitate successful completion of the random accessprocedure by link-budget-limited devices in the absence of knowledge ofwhich PRACH-transmitting devices are link-budget-limited and which arenot.

While the features described herein are susceptible to variousmodifications and alternative forms, specific embodiments thereof areshown by way of example in the drawings and are herein described indetail. It should be understood, however, that the drawings and detaileddescription thereto are not intended to be limiting to the particularform disclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the subject matter as defined by the appended claims.

DETAILED DESCRIPTION

Acronyms

Various acronyms are used throughout the present disclosure. Definitionsof the most prominently used acronyms that may appear throughout thepresent disclosure are provided below:

BS: Base Station

DL: Downlink

LBL: Link Budget Limited

LTE: Long Term Evolution

MIB: Master Information Block

NW: Network

PDCCH: Physical Downlink Control Channel

PDSCH: Physical Downlink Shared Channel

PRACH: Physical Random Access Channel

PUCCH: Physical Uplink Control Channel

PUSCH: Physical Uplink Shared Channel

RACH: Random Access Procedure or Random Access Channel.

RAR: Random Access Response

RA-RNTI: Random Access-Radio Network Temporary Identifier

RRC: Radio Resource Control

RRC IE: RRC Information Element

RX: Reception

SFN: System Frame Number

SIB: System Information Block

SIBn: System Information Block of Type n

TTI: Transmit Time Interval

TX: Transmission

UE: User Equipment

UL: Uplink

UMTS: Universal Mobile Telecommunication System

ZC sequence: Zadoff-Chu sequence

3GPP: Third Generation Partnership Project

Terminology

The following is a glossary of terms used in this disclosure:

Memory Medium—Any of various types of non-transitory memory devices orstorage devices. The term “memory medium” is intended to include aninstallation medium, e.g., a CD-ROM, floppy disks, or tape device; acomputer system memory or random access memory such as DRAM, DDR RAM,SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash,magnetic media, e.g., a hard drive, or optical storage; registers, orother similar types of memory elements, etc. The memory medium mayinclude other types of non-transitory memory as well or combinationsthereof. In addition, the memory medium may be located in a firstcomputer system in which the programs are executed, or may be located ina second different computer system which connects to the first computersystem over a network, such as the Internet. In the latter instance, thesecond computer system may provide program instructions to the firstcomputer for execution. The term “memory medium” may include two or morememory mediums which may reside in different locations, e.g., indifferent computer systems that are connected over a network. The memorymedium may store program instructions (e.g., embodied as computerprograms) that may be executed by one or more processors.

Carrier Medium—a memory medium as described above, as well as a physicaltransmission medium, such as a bus, network, and/or other physicaltransmission medium that conveys signals such as electrical,electromagnetic, or digital signals.

Programmable Hardware Element—includes various hardware devicescomprising multiple programmable function blocks connected via aprogrammable interconnect. Examples include FPGAs (Field ProgrammableGate Arrays), PLDs (Programmable Logic Devices), FPOAs (FieldProgrammable Object Arrays), and CPLDs (Complex PLDs). The programmablefunction blocks may range from fine grained (combinatorial logic or lookup tables) to coarse grained (arithmetic logic units or processorcores). A programmable hardware element may also be referred to as“reconfigurable logic”.

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), television system, grid computing system, or otherdevice or combinations of devices. In general, the term “computersystem” can be broadly defined to encompass any device (or combinationof devices) having at least one processor that executes instructionsfrom a memory medium.

User Equipment (UE) (or “UE Device”)—any of various types of computersystems devices which are mobile or portable and which perform wirelesscommunications. Examples of UE devices include mobile phones or smartphones (e.g., iPhone™, Android™-based phones, etc.), portable gamingdevices (e.g., Nintendo DS™, PlayStation Portable™, Gameboy Advance™,iPhone™), laptops, PDAs, portable Internet devices, music players, datastorage devices, other handheld devices, wearable devices (such as smartwatches), etc. In general, the term “UE” or “UE device” can be broadlydefined to encompass any electronic, computing, and/ortelecommunications device (or combination of devices) which is easilytransported by a user and capable of wireless communication.

Base Station—The term “Base Station” has the full breadth of itsordinary meaning, and at least includes a wireless communication stationinstalled at a fixed location and used to communicate as part of awireless telephone system or radio system.

Processing Element—refers to various elements or combinations ofelements. Processing elements include, for example, circuits such as anASIC (Application Specific Integrated Circuit), portions or circuits ofindividual processor cores, entire processor cores, individualprocessors, programmable hardware devices such as a field programmablegate array (FPGA), and/or larger portions of systems that includemultiple processors.

Channel—a medium used to convey information from a sender (transmitter)to a receiver. It should be noted that since characteristics of the term“channel” may differ according to different wireless protocols, the term“channel” as used herein may be considered as being used in a mannerthat is consistent with the standard of the type of device withreference to which the term is used. In some standards, channel widthsmay be variable (e.g., depending on device capability, band conditions,etc.). For example, LTE may support scalable channel bandwidths from 1.4MHz to 20 MHz. In contrast, WLAN channels may be 22 MHz wide whileBluetooth channels may be 1 MHz wide. Other protocols and standards mayinclude different definitions of channels. Furthermore, some standardsmay define and use multiple types of channels, e.g., different channelsfor uplink or downlink and/or different channels for different uses suchas data, control information, etc.

Band—The term “band” has the full breadth of its ordinary meaning, andat least includes a section of spectrum (e.g., radio frequency spectrum)in which channels are used or set aside for the same purpose.

Link Budget Limited—includes the full breadth of its ordinary meaning,and at least includes a characteristic of a wireless device (a UE) whichexhibits limited communication capabilities, or limited power, relativeto a device that is not link budget limited, or relative to devices forwhich a radio access technology (RAT) standard has been developed. A UEthat is link budget limited may experience relatively limited receptionand/or transmission capabilities, which may be due to one or morefactors such as device design, device size, battery size, antenna sizeor design, transmit power, receive power, current transmission mediumconditions, and/or other factors. Such devices may be referred to hereinas “link budget limited” (or “link budget constrained”) devices. Adevice may be inherently link budget limited due to its size, batterypower, and/or transmit/receive power. For example, a smart watch that iscommunicating over LTE or LTE-A with a base station may be inherentlylink budget limited due to its reduced transmit/receive power and/orreduced antenna size. Alternatively, a device may not be inherently linkbudget limited, e.g., may have sufficient size, battery power, and/ortransmit/receive power for normal communications over LTE or LTE-A, butmay be temporarily link budget limited due to current communicationconditions, e.g., a smart phone being at the edge of a cell, etc. It isnoted that the term “link budget limited” includes or encompasses powerlimitations, and thus a power limited device may be considered a linkbudget limited device.

Automatically—refers to an action or operation performed by a computersystem (e.g., software executed by the computer system) or device (e.g.,circuitry, programmable hardware elements, ASICs, etc.), without userinput directly specifying or performing the action or operation. Thusthe term “automatically” is in contrast to an operation being manuallyperformed or specified by the user, where the user provides input todirectly perform the operation. An automatic procedure may be initiatedby input provided by the user, but the subsequent actions that areperformed “automatically” are not specified by the user, i.e., are notperformed “manually”, where the user specifies each action to perform.For example, a user filling out an electronic form by selecting eachfield and providing input specifying information (e.g., by typinginformation, selecting check boxes, radio selections, etc.) is fillingout the form manually, even though the computer system must update theform in response to the user actions. The form may be automaticallyfilled out by the computer system where the computer system (e.g.,software executing on the computer system) analyzes the fields of theform and fills in the form without any user input specifying the answersto the fields. As indicated above, the user may invoke the automaticfilling of the form, but is not involved in the actual filling of theform (e.g., the user is not manually specifying answers to fields butrather they are being automatically completed). The presentspecification provides various examples of operations beingautomatically performed in response to actions the user has taken.

FIG. 1—Wireless Communication System

FIG. 1A illustrates one embodiment of a wireless communication system.It is noted that FIG. 1A represents one possibility among many, and thatfeatures of the present disclosure may be implemented in any of varioussystems, as desired.

As shown, the exemplary wireless communication system includes a basestation 102A which communicates over a transmission medium with one ormore wireless devices 106A, 106B, etc., through 106N. Wireless devicesmay be user devices, which may be referred to herein as “user equipment”(UE) or UE devices. Some of the wireless devices may be link budgetlimited (LBL) while others of the devices may be non-LBL.

The base station 102 may be a base transceiver station (BTS) or cellsite, and may include hardware that enables wireless communication withthe UE devices 106A through 106N. The base station 102 may also beequipped to communicate with a network 100 (e.g., a core network of acellular service provider, a telecommunication network such as a publicswitched telephone network (PSTN), and/or the Internet, among variouspossibilities). Thus, the base station 102 may facilitate communicationbetween the UE devices 106 and/or between the UE devices 106 and thenetwork 100.

The communication area (or coverage area) of the base station 102 may bereferred to as a “cell.” The base station 102 and the UEs 106 may beconfigured to communicate over the transmission medium using any ofvarious radio access technologies (RATs) or wireless communicationtechnologies, such as GSM, UMTS (WCDMA, TDS-CDMA), LTE, LTE-Advanced(LTE-A), HSPA, 3GPP2 CDMA2000 (e.g., 1×RTT, 1×EV-DO, HRPD, eHRPD),Wi-Fi, WiMAX etc.

Base station 102 and other similar base stations (not shown) operatingaccording to one or more cellular communication technologies may thus beprovided as a network of cells, which may provide continuous or nearlycontinuous overlapping service to UE devices 106A-N and similar devicesover a wide geographic area via one or more cellular communicationtechnologies.

Thus, while base station 102 may presently represent a “serving cell”for UE devices 106A-N as illustrated in FIG. 1A, each UE device 106 mayalso be capable of receiving signals from one or more other cells (e.g.,cells provided by other base stations), which may be referred to as“neighboring cells”. Such cells may also be capable of facilitatingcommunication between user devices and/or between user devices and thenetwork 100.

FIG. 1B illustrates an example of the base station 102 in communicationwith three wireless devices 106 ₁, 106 ₂ and 106 ₃, according to someembodiments. The wireless devices 106 ₁, 106 ₂ and 106 ₃ may be realizedby any combination of the wireless devices described above and/ordescribed below.

Note that at least in some instances a UE device 106 may be capable ofcommunicating using multiple wireless communication technologies. Forexample, a UE device 106 might be configured to communicate using two ormore of GSM, UMTS, CDMA2000, WiMAX, LTE, LTE-A, WLAN, Bluetooth, one ormore global navigational satellite systems (GNSS, e.g., GPS or GLONASS),one and/or more mobile television broadcasting standards (e.g., ATSC-M/Hor DVB-H), etc. Other combinations of wireless communicationtechnologies (including more than two wireless communicationtechnologies) are also possible. Likewise, in some instances a UE device106 may be configured to communicate using only a single wirelesscommunication technology.

FIG. 2 illustrates UE device 106 (e.g., one of the devices 106A through106N) in communication with base station 102. The UE device 106 may havecellular communication capability, and as described above, may be adevice such as a mobile phone, a hand-held device, a media player, acomputer, a laptop or a tablet, a wearable device (such as a smartwatch), or virtually any type of wireless device.

The UE device 106 may include a processor that is configured to executeprogram instructions stored in memory. The UE device 106 may perform anyof the method embodiments described herein by executing such storedinstructions. Alternatively, or in addition, the UE device 106 mayinclude a programmable hardware element such as an FPGA(field-programmable gate array) that is configured to perform any of themethod embodiments described herein, or any portion of any of the methodembodiments described herein.

In some embodiments, the UE device 106 may be configured to communicateusing any of multiple radio access technologies and/or wirelesscommunication protocols. For example, the UE device 106 may beconfigured to communicate using one or more of GSM, UMTS, CDMA2000, LTE,LTE-A, WLAN, Wi-Fi, WiMAX or GNSS. Other combinations of wirelesscommunication technologies are also possible.

The UE device 106 may include one or more antennas for communicatingusing one or more wireless communication protocols or technologies. Inone embodiment, the UE device 106 might be configured to communicateusing a single shared radio. The shared radio may couple to a singleantenna, or may couple to multiple antennas (e.g., for MIMO) forperforming wireless communications. Alternatively, the UE device 106 mayinclude two or more radios. For example, the UE 106 might include ashared radio for communicating using either of LTE or 1×RTT (or LTE orGSM), and separate radios for communicating using each of Wi-Fi andBluetooth. Other configurations are also possible.

FIG. 3—Example Block Diagram of a UE

FIG. 3 illustrates one possible block diagram of a UE 106. As shown, theUE 106 may include a system on chip (SOC) 300, which may includeportions for various purposes. For example, as shown, the SOC 300 mayinclude processor(s) 302 which may execute program instructions for theUE 106, and display circuitry 304 which may perform graphics processingand provide display signals to the display 340. The processor(s) 302 mayalso be coupled to memory management unit (MMU) 305, which may beconfigured to receive addresses from the processor(s) 302 and translatethose addresses to locations in memory (e.g., memory 306, read onlymemory (ROM) 350, NAND flash memory 310). The MMU 305 may be configuredto perform memory protection and page table translation or set up. Insome embodiments, the MMU 305 may be included as a portion of theprocessor(s) 302.

The UE 106 may also include other circuits or devices, such as thedisplay circuitry 304, radio 330, connector I/F 320, and/or display 340.

In the embodiment shown, ROM 350 may include a bootloader, which may beexecuted by the processor(s) 302 during boot up or initialization. Asalso shown, the SOC 300 may be coupled to various other circuits of theUE 106. For example, the UE 106 may include various types of memory(e.g., including NAND flash 310), a connector interface 320 (e.g., forcoupling to a computer system), the display 340, and wirelesscommunication circuitry (e.g., for communication using LTE, CDMA2000,Bluetooth, WiFi, GPS, etc.).

The UE device 106 may include at least one antenna, and in someembodiments multiple antennas, for performing wireless communicationwith base stations and/or other devices. For example, the UE device 106may use antenna 335 to perform the wireless communication. As notedabove, the UE may in some embodiments be configured to communicatewirelessly using a plurality of wireless communication standards.

As described herein, the UE 106 may include hardware and softwarecomponents for implementing any of the UE-related method embodimentsdescribed herein.

The processor 302 of the UE device 106 may be configured to implementpart or all of the methods described herein, e.g., by executing programinstructions stored on a memory medium (e.g., a non-transitorycomputer-readable memory medium). In other embodiments, processor 302may be configured as a programmable hardware element, such as an FPGA(Field Programmable Gate Array), or as an ASIC (Application SpecificIntegrated Circuit).

FIG. 4—Base Station

FIG. 4 illustrates one embodiment of a base station 102. It is notedthat the base station of FIG. 4 is merely one example of a possible basestation. As shown, the base station 102 may include processor(s) 404which may execute program instructions for the base station 102. Theprocessor(s) 404 may also be coupled to memory management unit (MMU)440, which may be configured to receive addresses from the processor(s)404 and translate those addresses to locations in memory (e.g., memory460 and read only memory (ROM) 450) or to other circuits or devices.

The base station 102 may include at least one network port 470. Thenetwork port 470 may be configured to couple to a telephone network andprovide a plurality of devices, such as UE devices 106, access to thetelephone network as described above.

The network port 470 (or an additional network port) may also oralternatively be configured to couple to a cellular network, e.g., acore network of a cellular service provider. The core network mayprovide mobility related services and/or other services to a pluralityof devices, such as UE devices 106. In some cases, the network port 470may couple to a telephone network via the core network, and/or the corenetwork may provide a telephone network (e.g., among other UE devicesserviced by the cellular service provider).

The base station 102 may include a radio 430, a communication chain 432and at least one antenna 434. The base station may be configured tooperate as a wireless transceiver and may be further configured tocommunicate with UE devices 106 via radio 430, communication chain 432and the at least one antenna 434. Communication chain 432 may be areceive chain, a transmit chain or both. The radio 430 may be configuredto communicate via various RATs, including, but not limited to, GSM,UMTS, LTE, WCDMA, CDMA2000, WiMAX, etc.

The processor(s) 404 of the base station 102 may be configured toimplement part or all of the methods described herein, e.g., byexecuting program instructions stored on a memory medium (e.g., anon-transitory computer-readable memory medium). Alternatively, theprocessor 404 may be configured as a programmable hardware element, suchas an FPGA (Field Programmable Gate Array), or as an ASIC (ApplicationSpecific Integrated Circuit), or a combination thereof.

PRACH Specifications in 3GPP

FIG. 5A illustrates a preamble 500 in a Physical Random Access Channel(PRACH) according to the existing LTE specifications. A UE devicetransmits the PRACH preamble in an uplink frame 510 in order to initiatethe random access procedure. (The uplink frame includes a plurality ofsubframes.) The time offset and frequency offset of the PRACH preamblewithin the uplink frame may be determined by the higher layer signaling.

FIG. 5B illustrates one particular realization of the PRACH preambleaccording to existing LTE specifications. In frequency, the PRACHpreamble (including the guard subcarriers at the beginning and end)spans 6 RBs=1.08 MHz. In time, the PRACH preamble, including the cyclicprefix (CP) and guard time (GT), spans one uplink subframe.

Formats 0-3 for the PRACH preamble each use a Zadoff-Chu sequence oflength 839, whereas format 4 uses a Zadoff-Chu sequence of length 139.

The PRACH preamble occupies 6 resource bocks (RBs) in uplink bandwidth(UL BW).

One PRACH subcarrier occupies 1.25 kHz whereas a normal UL subcarrieroccupies 15 kHz. The symbols of the Zadoff-Chu sequence are transmittedon respective ones of the PRACH subcarriers.

With respect to the PRACH preamble, FIG. 6 illustrates a cyclic prefix(CP) of duration T_(CP) and a sequence portion of duration T_(SEQ). (Thesequence portion contains the Zadoff-Chu sequence.) Table 1 below showsthe values of T_(CP) and T_(SEQ) in different formats of the PRACHpreamble.

TABLE 1 Random Access Preamble Parameters Preamble Format T_(CP) T_(SEQ)0  3168*Ts  24576*Ts 1 21024*Ts  24576*Ts 2  6240*Ts 2*24576*Ts 321024*Ts 2*24576*Ts 4  448*Ts   4096*TsSummary of RACH Procedure

The RACH procedure may involve a series of messages sent between the UEand the base station, as shown in FIG. 7.

In a first message (MSG1), the UE transmits the PRACH preamble to thebase station (i.e., eNodeB in the parlance of LTE). The PRACH preamblemay be configured according to one of the formats discussed above or anyother desired format.

In response to decoding the first message, the eNodeB transmits a secondmessage (MSG2). The second message may be referred to as a random accessresponse (RAR). The PDSCH of the RAR message may include an uplinkgrant. The uplink grant may identified uplink resources in the PUSCH forthe UE to transmit uplink data.

In response to decoding the second message, the UE may transmit a thirdmessage (MSG3). In the PUSCH of MSG3, the UE may transmit uplink data.The content of the third message may be different in different contexts,e.g., may depend on the purpose for which the RACH procedure has beeninvoked. For example, the third message may include an RRC Request,Scheduling Request (SR), etc.

In response to receiving the third message, the eNodeB may transmit afourth message (MSG4), e.g., a contention resolution message.

Channels in LTE

LTE uses various channels so that data can be transported across the LTEradio interface. These channels are used to segregate the differenttypes of data and allow them to be transported across the radio accessnetwork in an orderly fashion. The different channels effectivelyprovide interfaces to the higher layers within the LTE protocolstructure, and enable an orderly and defined segregation of the data.

There are three categories or types of LTE data channels as follows.

Physical channels: These are transmission channels that carry user dataand control messages.

Transport channels: The transport channels offer information transfer toMedium Access Control (MAC) and higher layers.

Logical channels: Provide services for the Medium Access Control (MAC)layer within the LTE protocol structure.

LTE defines a number of physical downlink channels to carry informationreceived from the MAC and higher layers. The LTE downlink comprises aPhysical Downlink Shared Channel (PDSCH) and a Physical Downlink ControlChannel (PDCCH). The PDSCH is the main data-bearing channel which isallocated to users on a dynamic and opportunistic basis. The PDCCHcarries control information that indicates how resources in the PDSCHare allowed to user devices.

LTE Random Access Configuration

In the random access procedure of LTE, there are many possible PRACHconfigurations. For each of the cells, the corresponding eNodeB willsignal one of the PRACH configurations (by broadcasting the index of thePRACH configuration in SIB2) to be used by devices attempting randomaccess in that cell. This PRACH configuration will identify the preambleformat and allowable time resources for devices in the cell to use whentransmitting the PRACH. The allowable time resources may includeallowable System Frame Numbers (SFNs) and allowable Subframe Numbers(SFNs). FIG. 8 shows the PRACH configurations defined in Table 5.7.1-2of 3GPP TS 36.211. Each PRACH configuration has a corresponding indexthat identified its positions with the list of PRACH configurations.

LTE Random Access Preambles

In LTE, the UEs in a given cell may initiate random access bytransmitting preambles selected from a set of preambles. (Each UE mayrandomly select a preamble from the set of preambles.) The UE generatesthe set of preambles using a root sequence number that is provided bythe eNodeB in SIB2. For a given cell, the set of preambles may includeup to 64 preambles, and the 64 preambles are divided into two groups:group A and group B.

The eNodeB also broadcasts (in SIB2) the following parameters toconfigure use of the preamble groups:

-   -   numberOfRA-Preambles is the number of non-dedicated random        access preambles available in the cell;    -   messageSizeGroupA is the threshold for preamble selection (in        bits);    -   sizeOfRA-PreamblesGroupA is the size (i.e., number) of random        access preambles in group A.        These parameters are included in RACH-ConfigCommon. The number        of preambles in group B is equal to the difference        ‘numberOfRA-Preambles’−‘sizeOfRA-PreamblesGroupA’.

According to the LTE specifications, a UE selects a preamble from groupB if the size of a potential UL message to be transmitted by the UE isgreater than ‘messageSizeGroupA’ and the pathloss is less than apathloss threshold.

The UE may measure pathloss based on RSCP. (RSCP is an acronym forReference Signal Code Power.) The network may broadcast the cellreference signal power transmitted at the eNodeB in system informationblocks (SIBs). The UE measures the received cell reference signal power.The difference between the cell reference signal power and the receivedvalue is path loss.

The pathloss threshold may be determined based on the expression:PathLossThreshold=P_(CMAX)−preambleInitialReceivedTargetPower−deltaPreambleMsg3−messagePowerOffsetGroupB,where

-   -   P_(CMAX) is UE maximum transmit power;    -   preambleInitialReceivedTargetPower is the network-expected        received power for first 1^(st) random access attempt (RACH        preamble);    -   deltaPreambleMsg3 is the power offset between the power of        preamble and Msg3, and    -   messagePowerOffsetGroupB is the power offset for preambles in        group B.

The following is the definition of the RACH-ConfigCommon informationelement according to LTE:

RACH-ConfigCommon ::= SEQUENCE {  preambleInfo SEQUENCE {  numberOfRA-Preambles ENUMERATED {            n4, n8, n12, n16, n20,n24, n28,            n32, n36, n40, n44, n48, n52, n56,            n60,n64},   preamblesGroupAConfig SEQUENCE {    sizeOfRA-PreamblesGroupAENUMERATED {             n4, n8, n12, n16, n20, n24, n28,            n32, n36, n40, n44, n48, n52, n56,             n60},   messageSizeGroupA ENUMERATED {b56, b144, b208, b256},   messagePowerOffsetGroupB ENUMERATED {             minusinfinity, dB0,dB5, dB8, dB10, dB12,             dB15, dB18},    ...   }  OPTIONAL  }Random Access—UE Identification at MSG1 (PRACH Message)

In one set of embodiments, the link-budget-limited device may beconfigured to transmit PRACH messages with a different PRACHconfiguration than the non-LBL devices. (The non-LBL devices may use theconventionally-signaled PRACH configuration.) Thus, the base station isable to determine whether or not a received PRACH message corresponds toan LBL device based on whether the PRACH message conforms to thedifferent PRACH configuration or the conventionally-signaled PRACHconfiguration. For a device that is determined to be LBL, the basestation may invoke one or more mechanisms to improve the likelihood ofsuccessful completion of the random access procedure for the LBL device.For example, the base station may boost the power of transmission ofmsg2 (i.e., the random access response message) for the LBL device.

In one embodiment, the eNodeB may broadcast an alternative PRACHconfiguration index for use by link-budget-limited devices, differentfrom the conventional PRACH configuration index of the current cell.(The alternative PRACH configuration index may also be different fromthe conventional PRACH configuration indices of neighboring cells.) Whenthe LBL devices transmit PRACH messages, they may use the PRACHconfiguration identified by the alternative PRACH configuration index.(The conventional PRACH configuration index is signaled in SIB2.However, the alternative PRACH configuration index may be included in aspecial SIB for the LBL UE devices, i.e., a new SIB created tofacilitate the broadcast of RACH configuration information for the LBLdevices.) In contrast, when the non-LBL devices transmit PRACH messages,they use the PRACH configuration corresponding to the conventional PRACHconfiguration index.

In another embodiment, the LBL devices may be configured to employ analternative PRACH configuration index which is displaced from theconventional PRACH configuration index of the current cell by apredetermined offset. The LBL devices in the cell may transmit PRACHmessages according to the PRACH configuration identified by thealternative PRACH index while the non-LBL devices in the cell transmitPRACH messages according to the PRACH configuration identified by theconventional PRACH configuration index. Each LBL device may receive theconventional PRACH configuration index from the base station (e.g., fromSIB2), and compute the alternative index by adding the predeterminedoffset. The base station may determine whether or not a given PRACHmessage corresponds to an LBL device by determining whether the PRACHformat, the SFN and the Subframe Number of the PRACH message areconsistent with the alternative PRACH configuration or the conventionalPRACH configuration.

In yet another embodiment, the LBL devices may use the preamble formatand the SFNs defined by the conventionally signaled PRACH configuration,but use a set of allowable subframes that is different from theallowable subframe set defined by the conventionally signaled PRACHconfiguration. Thus, the LBL devices transmit PRACH messages in the sameframes as non-LBL devices, but in different subframes. For example,according to PRACH configuration 6, only subframes 1 and 6 areconventionally allowed for PRACH transmission. (See the Table in FIG.8.) In this case, the eNodeB may allow the LBL devices to use subframes0, 2-5, 7-9 (or any subset of those subframes) for transmission of PRACHmessages. The one or subframes available for use by the LBL devices maybe broadcast in a special SIB.

In yet another embodiment, if the conventionally-signaled PRACHconfiguration of a given cell specifies the use of the even SFNs (e.g.,configurations corresponding to indices 0-2 and 15-18) for PRACH messagetransmission, the LBL devices may instead use the odd SFNs, but respectthe preamble format and allowed subframes defined by theconventionally-signaled PRACH configuration. As an example, when theconventionally-signaled PRACH configuration index is 2, the LBL devicestransmit their PRACH messages in odd frames, in subframe 7, and withpreamble format 0.

In one set of embodiments, the set of available preambles in a cell maybe expanded to include additional preambles beyond the conventional setof preambles. The additional preambles may be used by the LBL devices totransmit PRACH messages while the preambles of the conventional set areused by non-LBL devices. For example, in one embodiment, an LBL devicemay generate an expanded set of 128 preambles using the root sequencenumber provided by the eNodeB. The expanded set of 128 preambles mayinclude the 64 preambles of the conventional set and 64 additionalpreambles. The LBL device may select (e.g., randomly select) a preamblefrom the 64 additional preambles. Because the LBL devices and non-LBLdevices use disjoint sets of preambles to transmit their PRACH messages,the base station can easily determine whether or not a given PRACHmessage corresponds to an LBL device by determining the set membership(conventional set or LBL-specific set) of the preamble contained in thePRACH message. While the above discussion given an example where thenumber of additional preambles is 64, it should be understood that thisnumber could take any of a wide variety of values, e.g., depending onthe expected or average number of LBL devices in the cell. In someembodiments, the number of additional preambles is dynamicallyconfigurable, e.g., based on system information that is broadcast by thebase station.

In one set of embodiments, at least a subset of the preambles of group Bmay be reserved for PRACH transmissions (orconnection-establishment-related PRACH transmissions) bylink-budget-limited devices. (See the above discussion of groups A andB.) The eNodeB may configure the cell so that a desired number ofpreambles are allocated to group B, and may relax the pathlossthreshold. (The pathloss threshold may be relaxed by setting thepathloss threshold to a value sufficiently larger to ensure, or increasethe likelihood that, LBL UE devices will pass the pathloss test to usegroup B. Note that LBL UE devices typically have higher values of pathloss than non-LBL devices.) Alternatively or in addition, the eNodeB mayset ‘messageSizeGroupA’ such that non-LBL devices will not use the groupB preambles (or use the group B preambles infrequently).

In some embodiments, the eNodeB may determine whether a receivedpreamble (i.e., the preamble of a received PRACH message) belongs togroup A or group B. If the preamble belongs to group B, the eNodeB mayidentify the UE device that transmitted the preamble as being an LBLdevice, and transmit to the UE device a random access response messagegranting enough uplink resources to the UE device for connectionestablishment, though the granted uplink resources may be of total sizeless than the ‘messageSizeGroupA’. Also, after detecting that thepreamble belongs to group B, the eNodeB may boost the power of messages(MSG2 and following messages) to the LBL device.

In one set of embodiments, a method 900 for operating a user equipment(UE) device may be performed as illustrated in FIG. 9. (Method 900 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-8 and described below inconnection with FIGS. 10-17.) The method 900 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements suchas FPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing.

While method 900 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 910, the processing agent may receive an index I_(LBL) that has beenbroadcast by a base station. The index I_(LBL) identifies a firstconfiguration for transmission of a Physical Random Access Channel(PRACH) by link-budget-limited UE devices in a cell corresponding to thebase station. The index I_(LBL) is different from an index I_(NLBL) thatis also broadcast by the base station, i.e., the index I_(LBL) is notequal to the index I_(NLBL) in terms of numeric value. The indexI_(NLBL) identifies a second configuration for transmission of the PRACHby non-link-budget-limited UE devices (and/or legacy devices) in thecell, where the second configuration is different from the firstconfiguration.

At 915, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station, wherein the PRACH is transmittedaccording to the first configuration. (In contrast, in response to adetermination—e.g., at a later time—that the UE device has beenclassified as not being link budget limited, the UE device may performPRACH transmission according to the second configuration.)

In some embodiments, the base station may be configured to broadcast afirst system information block and a second system information block,wherein the first information block includes the index I_(LBL), and thesecond system information block includes the index I_(NLBL). Thelink-budget-limited UE device may read the first SIB to determine theindex I_(LBL). More generally, each UE device that is link budgetlimited may recover the index I_(LBL) from the first SIB. (In contrast,each UE device that is not link budget limited may recover the indexI_(NLBL) from the second SIB.)

In some embodiments, the second system information block is a systeminformation block of type 2, e.g., as defined by the LTE standard.

In some embodiments, the first configuration specifies a first set ofallowable temporal opportunities (e.g., allowable subframes) fortransmission of the PRACH by link-budget-limited UE devices, and thesecond configuration specifies a second set of allowable temporalopportunities for transmission of the PRACH by non-link-budget-limitedUE devices, where the first set and the second set are disjoint sets.

In some embodiments, the index I_(LBL) identifies the firstconfiguration from a list of PRACH configurations defined by an existingwireless communication standard, and the index I_(NLBL) identifies thesecond configuration from the same list. (For example, the list ofconfigurations may be the list of PRACH configurations specified in 3GPPTS 36.211.) The UE device may store a copy of the list, and access thefirst configuration from the list based on the index I_(LBL). When theUE device is not link budget limited, e.g., at a later time, the UEdevice may access the second configuration from the list based on theindex I_(NLBL), and perform PRACH transmission using the secondconfiguration instead of using the first configuration.

As noted above, the index I_(LBL) is different from the index I_(NLBL).In some embodiments, the index I_(LBL) is also different from one ormore additional PRACH configuration indices that are broadcastrespectively by one or more neighboring base stations, where the one ormore additional PRACH configuration indices are broadcast for use bynon-link-budget-limited UE devices in one or more additional cellscorresponding respectively to the one or more additional base stations.In some embodiments, each of a plurality of neighboring base stationsmay be configured to perform method 900. The base stations may beconfigured so that each base station selects its index I_(LBL) so as tobe different from its index I_(NLBL) and to be different from the indexI_(LBL) and the index I_(NLBL) transmitted by the other base stations.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-9 and described below in connection with FIGS. 10-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast an index I_(LBL). The index I_(LBL)identifies a first configuration for transmission of a Physical RandomAccess Channel (PRACH) by link-budget-limited UE devices in a cellcorresponding to the base station. The index I_(LBL) is different froman index I_(NLBL) that is also broadcast by the base station, i.e., theindex I_(LBL) is not equal to the index I_(NLBL) in terms of numericvalue. The index I_(NLBL) identifies a second configuration fortransmission of the PRACH by non-link-budget-limited UE devices (and/orlegacy devices) in the cell, where the second configuration is differentfrom the first configuration.

The processing agent may receive a PRACH, i.e., a PRACH that has beentransmitted by a UE device. The processing agent may determine whetherthe PRACH has been transmitted according to the first configuration oraccording to the second configuration. The first configuration and thesecond configuration may have different preamble formats, and/or,different allowed system frame numbers, and/or different allowedsubframe numbers. Any or all such differences may be used as the basisfor determining whether the PRACH has been transmitted according to thefirst configuration or according to the second configuration.

In response to determining that the PRACH has been transmitted accordingto the first configuration, the processing agent may invoke one or morecommunication enhancement mechanisms for transmission to and/orreception from the UE device that transmitted the PRACH. (For example,the processing agent may boost the power of the random access responsemessage, and/or, boost the power of downlink traffic transmissions tothe UE device.) In contrast, if the processing agent determines that thePRACH has been transmitted according to the second configuration, theprocessing agent may not invoke the one or more communicationenhancement mechanisms.

In one set of embodiments, a method 1000 for operating a user equipment(UE) device may be performed as illustrated in FIG. 10. (Method 1000 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-9 and described below inconnection with FIGS. 11-17.) The method 1000 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1000 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1010, the processing agent may receive a first index that has beenbroadcast by a base station, wherein the first index identifies a firstconfiguration for transmission of a Physical Random Access Channel(PRACH) by non-link-budget-limited UE devices in a cell of the basestation. The first index may be the PRACH configuration index that isconventionally signaled by the eNodeB of LTE. In other words, the firstindex may be a PRACH configuration index conforming to the LTE wirelesscommunication standard, e.g., as defined in 3GPP TS 36.211.

At 1015, the processing agent may add an offset (i.e., a non-zerooffset) to the first index in order to obtain a second index, whereinthe second index identifies a second configuration for transmission ofthe PRACH by link-budget-limited UE devices in the cell. The secondconfiguration is different from the first configuration.

At 1020, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit a PRACH to the base station, wherein the PRACH is transmittedaccording to the second configuration. (In contrast, in response to adetermination—e.g., at a later time—that the UE device has beenclassified as being not link budget limited, the UE device may performPRACH transmission according to the first configuration, i.e., theconfiguration identified by the first index.) In some embodiments, thefirst configuration specifies a first set of allowable temporalopportunities (e.g., allowable subframes) for transmission of the PRACH,and the second configuration specifies a second set of allowabletemporal opportunities for transmission of the PRACH, where the firstset and the second set are disjoint sets.

In some embodiments, the base station is configured to broadcast thefirst index as part of SIB2 (i.e., the system information block of type2), as defined by specifications in 3GPP TS 36.331.

In some embodiments, the first index identifies the first configurationfrom a list of PRACH configurations defined by an existing wirelesscommunication standard, and the second index identifies the secondconfiguration from the same list of PRACH configurations. (For example,the list of PRACH configurations may be the list defined by 3GPP TS36.211.) In some embodiment, the above described addition of the offsetis an addition modulo N_(LIST), where N_(LIST) is the number of PRACHconfigurations in said list.

In some embodiments, the method 1000 also includes: (a) receiving athird index that has been broadcast by a second base station, where thethird index identifies a third configuration for transmission of thePRACH by non-link-budget-limited UE devices in a second cellcorresponding to the second base station; (b) adding the same offset(i.e., the offset of step 1015) to the third index to obtain a fourthindex, wherein the fourth index identifies a fourth configuration fortransmission of the PRACH by link-budget-limited UE devices in thesecond cell; and (c) transmitting the PRACH to the second base station,wherein this PRACH is transmitted according to the fourth configuration.

In some embodiments, the offset may be used by a plurality of basestations including said base station.

In some embodiments, each of a plurality of neighboring base stationsmay perform method 1000. Different ones of the base stations maytransmit respectively different values of the first index, but use thesame offset to determine their respective values of the second index.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-10 and described below in connection with FIGS. 11-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast an index I_(CONV), where the indexI_(CONV) identifies a first configuration for transmission of a PhysicalRandom Access Channel (PRACH) by non-link-budget-limited UE devices(and/or legacy devices) in a cell of the base station. The indexI_(CONV) may be the PRACH configuration index that is conventionallysignaled by LTE base stations. Non-link-budget limited UE devices and/orlegacy UE devices in the cell may receive the index I_(CONV), andperform PRACH transmission using the first configuration. In contrast,link-budget-limited UE devices in the cell may perform PRACHtransmission using a second configuration that is offset from the firstconfiguration by a known distance in a list of allowed configurations,i.e., a distance that is known to the base station and thelink-budget-limited UE devices. See, e.g., FIG. 8.

After having broadcast the index I_(CONV), the processing agent mayreceive a PRACH, i.e., a PRACH that has been transmitted by a UE device.

The processing agent may analyze the PRACH to determine whether it hasbeen transmitted according to the first configuration or according tothe second configuration. The first configuration and the secondconfiguration may have different preamble formats, and/or, differentallowed system frame numbers, and/or different allowed subframe numbers.Any or all such differences may be used as the basis for determiningwhether the PRACH has been transmitted according to the firstconfiguration or according to the second configuration.

In response to determining that the PRACH has been transmitted accordingto the second configuration, the processing agent may invoke one or morecommunication enhancement mechanisms for transmission to and/orreception from the UE device that transmitted the PRACH. (For example,the processing agent may boost the power of the random access responsemessage, and/or, boost the power of downlink traffic transmissions tothe UE device.) In contrast, if the processing agent determines that thePRACH has been transmitted according to the first configuration, theprocessing agent may not invoke the one or more communicationenhancement mechanisms.

In one set of embodiments, a method 1100 for operating a user equipment(UE) device may be performed as illustrated in FIG. 11. (Method 1100 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-10 and described below inconnection with FIGS. 12-17.) The method 1100 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1100 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1110, the processing agent may receive an index that has beenbroadcast by a base station, wherein the index identifies aconfiguration for transmission of a Physical Random Access Channel(PRACH), wherein the configuration has a conventionally-defined set ofallowable subframes for transmission of the PRACH. Theconventionally-defined set may be used for PRACH transmission bynon-link-budget limited UE devices and/or legacy devices in a cellcorresponding to the base station. The conventionally-defined set may bethe set of allowable subframes defined by a wireless communicationstandard such as LTE. For example, the conventionally-defined set may bethe set of allowable subframes corresponding to a selected one of thePRACH configurations defined in 3GPP TS 36.211. (See Table 5.7.1-2 ofthat technical specification.) The above described index may be an indexto Table 5.7.1-2 of 3GPP TS 36.211.

At 1115, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station. The PRACH may be transmitted ina subframe from an alternative set of allowable subframes, wherein thealternative set is disjoint from the conventionally-defined set ofallowable subframes. In some embodiments, the UE device may randomlyselect a subframe from the alternative set. The alternative set ofallowable subframes may be reserved for use by link budget limited UEdevices. The link budget limited UE devices may be designed to use thealternative set (as opposed to the conventionally-defined set) whenselecting a subframe for PRACH transmission.

In some embodiments, the base station is configured to broadcast systeminformation identifying the alternative set of allowable subframes.

The base station may be configured to monitor the alternative set ofallowable subframes for PRACH transmissions from link-budget-limited UEdevices, and to monitor the conventionally-defined set of allowablesubframes for PRACH transmissions from non-link-budget-limited UEdevices and/or legacy UE devices. The ability for thelink-budget-limited UE devices to perform PRACH transmissions usingallowable subframes disjoint from those used by non-LBL devices and/orlegacy devices allows the base station to receive the PRACHtransmissions from the link-budget-limited UE device(s) with a lowerinterference, and thus, to decode those PRACH transmissions with lowererror probability.

In some embodiments, the base station is configured to broadcast theindex as part of a system information block of type 2, as defined by theLTE standard.

In some embodiments, the method 1100 may also include: in response to adetermination that the UE device has been classified as not beinglink-budget-limited, transmitting another PRACH to the base station,wherein the other PRACH is transmitted in a subframe from theconventionally-defined set of allowable subframes.

In some embodiments, the configuration for transmission of the PRACHalso specifies a format for transmission of the PRACH, e.g., one of thePRACH formats defined in 3GPP TS 36.211. The above described PRACH(i.e., the PRACH of step 1115) may be transmitted according to thespecified format.

In some embodiments, the configuration for transmission of the PRACHalso specifies a constraint on allowable frames, e.g., as shown in FIG.8. The above described PRACH may be transmitted in a frame conforming tothe allowable frames constraint.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-11 and described below in connection with FIGS. 12-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast an index, wherein the indexidentifies a configuration for transmission of a Physical Random AccessChannel (PRACH). The configuration may have (be associated with) aconventionally-defined set of allowable subframes for transmission ofthe PRACH. The conventionally-defined set may be used for PRACHtransmission by non-link-budget limited UE devices (and/or legacydevices) in a cell corresponding to the base station. (Theconventionally-defined set may be the set of allowable subframes definedby a wireless communication standard such as LTE.) In contrast,link-budget-limited UE devices may be configured to perform PRACHtransmissions using an alternative set of allowable subframes, disjointfrom the conventionally-defined set.

In addition to having a conventionally-defined set of allowablesubframes, the PRACH configuration may identify a preamble format andspecify a constraint on allowable frames for PRACH transmission. (See,e.g., FIG. 8.) When performing PRACH transmissions, thelink-budget-limited UE devices may use the identified preamble formatand use frames conforming to the allowable frames constraint.

After having broadcast the index, the processing agent may receive aPRACH, i.e., a PRACH that has been transmitted by a UE device. Theprocessing agent may determine whether the PRACH has been transmitted inone of the subframes of alternative set or one of the subframes of theconventionally-defined set.

In response to determining that the PRACH has been transmitted in one ofthe subframes of the alternative set, the processing agent may invokeone or more communication enhancement mechanisms for transmission toand/or reception from the UE device that transmitted the PRACH. (Forexample, the processing agent may boost the power of the random accessresponse message, and/or, boost the power of downlink traffictransmissions to the UE device.) In contrast, if the processing agentdetermines that the PRACH has been transmitted in one of the subframesof the conventionally-defined set, the processing agent may not invokethe one or more communication enhancement mechanisms.

In one set of embodiments, a method 1200 for operating a user equipment(UE) device may be performed as illustrated in FIG. 12. (Method 1200 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-11 and described below inconnection with FIGS. 13-17.) The method 1200 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1200 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1210, the processing agent may receive an index that has beenbroadcast by a base station. The index identifies a first configurationfrom a list of configurations for transmission of a Physical RandomAccess Channel (PRACH). Each of the configurations in the list may have:

-   -   a corresponding conventionally-defined set of allowable        subframes for transmission of the PRACH by non-link-budget        limited UE devices (and/or legacy devices) in a cell        corresponding to the base station; and    -   a corresponding alternative set of allowable subframes for        transmission of the PRACH by link-budget-limited UE devices in        the cell, wherein the corresponding alternative set is disjoint        from the corresponding conventionally-defined set.        The list of configurations may be stored in a memory of the UE        device.

At 1215, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station. The PRACH may be transmitted inone of the subframes from the alternative set of allowable subframescorresponding to the index. If the status of the UE device changes tobeing not link budget limited, the UE device may transmit a PRACH in oneof the subframes of the conventionally-defined set.

In one set of embodiments, a method 1300 for operating a user equipment(UE) device may be performed as illustrated in FIG. 13. (Method 1300 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-12 and described below inconnection with FIGS. 14-17.) The method 1300 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1300 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1310, the processing agent may receive an index that has beenbroadcast by a base station. The index may identify a configuration fortransmission of a Physical Random Access Channel (PRACH). Theconfiguration may specify that transmission of the PRACH bynon-link-budget-limited UE devices (and/or legacy devices) is restrictedto even-numbered frames and to a conventionally-defined set of allowablesubframes. (Uplink frames are consecutively numbered, e.g., using asystem frame number.)

At 1315, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station, wherein the PRACH is transmittedin an odd-numbered frame and in one of the subframes from theconventionally-defined set. By using an odd-numbered frame, thelink-budget-limited UE device avoids the even-numbered frames, which areused by the non-link-budget-limited UE devices for PRACH transmissions.This strategy allows the base station to receive the PRACH transmissionfrom the link-budget-limited UE device without interference from PRACHtransmissions of non-LBL UE devices (and/or legacy devices), and thus,to decode the PRACH transmission from the link-budget-limited UE devicewith lower probability of error. The base station may be configured tomonitor the even-numbered frames for PRACH transmissions from non-LBL UEdevices (and/or legacy device), and to monitor the odd-numbered framesfor PRACH transmissions from link-budget-limited UE devices.

In some embodiments, the configuration also specifies a format for PRACHtransmission, e.g., one of the formats defined in the LTE standard andreferred to in the “Preamble Format” column of Table 5.7.1-2 of 3GPP TS36.211. (A copy of that Table is given in FIG. 8.) The transmission ofthe PRACH to the base station, i.e., the transmission of step 1315, maybe performed according to said format. The index may be interpreted asthe PRACH configuration index of the Table. Each value of the PRACHconfiguration index may identify a corresponding PRACH configurationincluding one or more of the following: a corresponding preamble formatvalue, a corresponding constraint on system frame number (e.g., aselection of “even” or “any”), and a correspondingconventionally-defined set of allowable subframes.

In some embodiments, the method 1300 may also include: in response to adetermination that the UE device has been classified as not beingnon-link budget limited, transmitting another PRACH to the base station,wherein the other PRACH is transmitted in an even-numbered frame and inone of the subframes from the conventionally-defined set.

In some embodiments, the base station is configured to broadcast theindex as part of SIB2 (i.e., the system information block of type 2), asdefined by the LTE standard.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-12 and described below in connection with FIGS. 13-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast an index. The index may identify aconfiguration for transmission of a Physical Random Access Channel(PRACH). The configuration may specify (or indicate) that transmissionof the PRACH by non-link-budget-limited UE devices (and/or legacydevices) is restricted to even-numbered frames and to aconventionally-defined set of allowable subframes.

After having broadcast the index, the processing agent may receive aPRACH, i.e., a PRACH that has been transmitted by a UE device. Theprocessing agent may determine whether the PRACH has been transmitted inan even-numbered frame or an odd-numbered frame. (The processing agentmay maintain a system frame number that is incremented from one uplinkframe to the next.)

In response to determining that the PRACH has been transmitted in anodd-numbered frame, the processing agent may invoke one or morecommunication enhancement mechanisms for transmission to and/orreception from the UE device that transmitted the PRACH. (For example,the processing agent may boost the power of the random access responsemessage, and/or, boost the power of downlink traffic transmissions tothe UE device.) In contrast, if the processing agent determines that thePRACH has been transmitted in an even-numbered frame, the processingagent may not invoke the one or more communication enhancementmechanisms.

Background Regarding the Conventional PRACH Sequence Set

In 3GPP TS 36.211, a list of logical root sequence numbers andcorresponding physical root sequence numbers is specified for thePhysical Random Access Channel (PRACH). See Table 5.7.2-4 of TS 36.211.The eNodeB signals a logical root sequence number in SIB2. A UEgenerates a set of 64 Zadoff-Chu sequences based on: parameter Ncs (alsosignaled in SIB2); and physical root sequence numbers correspondingrespectively to consecutive logical root sequence numbers, starting withthe signaled logical root sequence number. In particular, the eNodeBgenerates a first subset of the Zadoff-Chu sequences based on cyclicshifts of a first root sequence (corresponding to the first physicalroot sequence number) until the first root sequence is exhausted, thengenerates a second subset of the Zadoff-Chu sequences based on cyclicshifts of a second root sequence (corresponding to the second physicalroot sequence number) until the second root sequence is exhausted, andso on, until 64 sequences have been generated. According to TS 36.211,the root sequence corresponding to physical root sequence number u isgiven by:

${{x_{u}(n)} = {\exp\left( {{- j}\frac{\pi\;{n\left( {n + 1} \right)}}{N_{ZC}}} \right)}},{0 \leq n \leq {N_{ZC} - 1}},$wherein the length N_(ZC) of the root sequence x_(u) is given by Table5.7.2.1 of TS 36.211.Special Preambles Used for Signaling LBL Status of UE Device

In one set of embodiments, a method 1400 for operating a user equipment(UE) device may be performed as illustrated in FIG. 14. (Method 1400 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-13 and described below inconnection with FIGS. 15-17.) The method 1400 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements, by one ormore dedicated hardware devices such as ASICs, or by any combination ofthe foregoing.

While method 1400 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1410, the processing agent may receive a logical root sequence numberthat has been broadcast by a base station.

At 1415, the processing agent may generate a set of preambles based ondata including the logical root sequence number. (The data may alsoinclude the parameter Ncs described below.) The action of generating theset of preambles may include determining a first physical root sequencenumber according to a conventional mapping of logical root sequencenumbers to physical root sequence numbers. (In the context of LTE, theconventional mapping may be the mapping defined by Table 5.7.2.1 of 3GPPTS 36.211.) The set of preambles may include:

-   -   a first subset of preambles for transmission of a Physical        Random Access Channel (PRACH) by non-link-budget-limited UE        devices; and    -   a second subset of preambles for transmission of the PRACH by        link-budget-limited UE devices, where the first subset and the        second subset are disjoint subsets.

In some embodiments, the set of preambles may be generated by applyingthe sequence generation procedure of 3GPP TS 36.211 Section 5.7.2, butextending that procedure so that more than 64 preambles are generated.For example, the first 64 preambles may form the first subset, and theremaining preambles may form the second sub set.

At 1420, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station using one of the preambles fromthe second subset.

In some embodiments, the processing agent may generate only the secondsubset of preambles. For example, if the UE device is link budgetlimited, it may omit the action of generating the first subset ofpreambles.

The base station may be configured so that, whenever it receives aPRACH, the base station may determine whether the preamble contained inthe received PRACH belongs to the first subset or the second subset,e.g., by performing correlation of the received PRACH against preamblesin the first subset and against preambles in the second subset. Thesubset membership of the correlation-maximizing preamble indicateswhether the UE device that transmitted the PRACH is link budget limitedor not. Thus, the UE device of method 1400 may signal itslink-budget-limited status to the base station by transmitting the PRACHusing a preamble selected from the second subset as opposed to apreamble from the first subset. Upon determining that the UE device islink budget limited, the base station may invoke one or more enhancementmechanisms for improving the reliability of uplink and/or downlinkcommunication with the UE device, e.g., mechanisms such as: employingmore complex decoding algorithms for decoding uplink transmissions fromthe UE device; transmitting downlink transmissions to the UE device withincreased power; etc. (A non-LBL UE device may transmit the PRACH usinga preamble selected from the first subset. Thus, the base station canrecognize that the device is not link budget limited.)

In some embodiments, the base station may also be configured tobroadcast the logical root sequence number and a parameter Ncs as partof a system information broadcast.

In some embodiments, the first subset of preambles is the set ofpreambles defined by the existing LTE specifications.

In some embodiments, the method 1400 may also include: in response to adetermination that the UE device has been classified as not being linkbudget limited, transmitting the PRACH to the base station using one ofthe preambles from the first subset. (The LBL status of the UE devicemay vary as the UE device moves within a cell, as battery power of theUE device is depleted over time or replenished upon charging, as otherobjects in the physical environment of the UE device change over time,etc.)

In some embodiments, the UE device may randomly select said preamblefrom the second subset of preambles.

In some embodiments, the set of preambles may be generated by: (1)generating a sequence of physical root sequence numbers based on thereceived logical root sequence number; and (2) applying cyclic shifts toroot sequences that correspond to the physical root sequence numbers ofsaid sequence. The sequence of physical root sequence numbers may begenerated by mapping consecutive logical root sequence numbers, startingwith the signaled logical root sequence number, to respective physicalroot sequence numbers using the conventional mapping.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-14 and described below in connection with FIGS. 15-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast a logical root sequence number.Non-link-budget-limited UE devices (and/or legacy devices) may use thelogical root sequence number to generate a first subset of preambles forPRACH transmission, e.g., according to a conventionally-definedalgorithm. In contrast, UE devices that are link budget limited maygenerate a second subset of preambles for PRACH transmission, where thesecond subset is disjoint from the first subset.

After having broadcast the logical root sequence number, the processingagent may receive a PRACH, i.e., a PRACH that has been transmitted by aUE device. The processing agent may determine whether the PRACH has beentransmitted using one of the preambles of the first subset or one of thepreambles of the second subset.

In response to determining that the PRACH has been transmitted using oneof the preambles of the second subset, the processing agent may invokeone or more communication enhancement mechanisms for transmission toand/or reception from the UE device that transmitted the PRACH. (Forexample, the processing agent may boost the power of the random accessresponse message, and/or, boost the power of downlink traffictransmissions to the UE device.) On the other hand, if the processingagent determines that the PRACH has been transmitted using one of thepreambles of the first subset, the processing agent may not invoke theone or more communication enhancement mechanisms.

In one set of embodiments, a method 1500 for operating a user equipment(UE) device may be performed as illustrated in FIG. 15. (Method 1500 mayalso include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-14 and described below inconnection with FIGS. 16-17.) The method 1500 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements, by one ormore dedicated hardware devices such as ASICs, or by any combination ofthe foregoing.

While method 1500 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1510, the processing agent may receive system information that hasbeen broadcast by a base station. The system information may include oneor more of the following elements:

-   -   a logical root sequence number;    -   a total number n_(TOTAL) of preambles included in a set of        preambles, wherein the set of preambles includes a first group        of preambles and a second group of one or more preambles,        wherein the first group and the second group are disjoint (as        subsets of the set of preambles); and    -   the number n₁ of preambles in the first group.        The number n₂ is positive but smaller than the number n_(TOTAL):        0<n ₁ <n _(TOTAL).

At 1515, the processing agent may generate the set of preambles based ondata including the logical root sequence number. The action ofgenerating the set of preambles may include generating the first groupof preambles with size equal to the number n₁, and generating the secondgroup of one or more preambles with size equal to a difference betweenthe number n_(TOTAL) and the number n₁. The first group of preambles maybe reserved for Physical Random Access Channel (PRACH) transmissions bynon-link-budget-limited UE devices (and/or legacy devices). The secondgroup of one or more preambles may be used for PRACH transmissions bylink-budget-limited UE devices.

At 1520, in response to a determination that the UE device has beenclassified as being link budget limited, the processing agent maytransmit the PRACH to the base station using one of the one or morepreambles from the second group.

When the base station receives a PRACH, the base station may determinewhether the preamble contained in the received PRACH belongs to thefirst group or the second group, e.g., by correlating the received PRACHagainst the preambles in the first group and against the one or morepreambles in the second group. The group membership (first group orsecond group) indicates whether the UE device that transmitted the PRACHis link budget limited or not. In response to determining that a givenUE device is link budget limited (based on the group membership), thebase station may invoke one or more communication enhancement mechanismsto improve the reliability of uplink and/or downlink communications withthe UE device.

In some embodiments, the above-described system information may includea RACH-ConfigCommon information element conforming to the LTE standard.The RACH-ConfigCommon message may include the number n_(TOTAL), thenumber n₁ and a message size. The base station may be configured to setthe message size equal to a value sufficiently large so as to decrease anumber of non-link-budget-limited UE devices that use preambles from thesecond group (or, so as to decrease the probability thatnon-link-budget-limited UE devices will use preambles from the secondgroup).

In some embodiments, the system information may include aRACH-ConfigCommon message conforming to the LTE standard. TheRACH-ConfigCommon message may include the number n_(TOTAL), the numbern₁ and a power offset. The base station may be configured to set thepower offset to a value sufficiently large so as to increase a number oflink-budget-limited UE devices that use preambles from the second group(or, to increase the probability that link-budget-limited UE deviceswill use preambles from the second group and not preambles from thefirst group).

In some embodiments, the system information may also include a parameterNcs, wherein the data of step 1515 includes the parameter Ncs.

In some embodiments, the system information may include aRACH-ConfigCommon message conforming to the LTE standard. TheRACH-ConfigCommon message may include the number n_(TOTAL), the numbern₁, an indication of a power offset, and an indication of a message sizethreshold. In these embodiments, the method 1500 may also include thefollowing. In response to a determination that the UE device has beenclassified as not being link budget limited, the processing agent mayperform operations including:

-   -   determining if (a) a path loss measured by the UE device is less        than a path loss threshold determined in part from the power        offset and (b) a size of an uplink message to be transmitted by        the UE device is greater than the message size threshold;    -   in response to determining that (a) and (b) are true, selecting        a preamble from the first group; and    -   transmitting a PRACH including the selected preamble.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-15 and described below in connection with FIGS. 16-17.) Themethod may be performed by a base station to facilitate a random accessprocedure for a link-budget-limited UE device. The method may beimplemented by a processing agent of the base station. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements such asFPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may broadcast system information. The systeminformation may include one or more of the following elements:

-   -   a logical root sequence number;    -   a total number n_(TOTAL) of preambles included in a set of        preambles, wherein the set of preambles includes a first group        of preambles and a second group of one or more preambles,        wherein the first group and the second group are disjoint; and    -   the number n₁ of preambles in the first group.

The UE device may generate a set of preambles based on data includingthe logical root sequence number. The action of generating the set ofpreambles may include generating the first group of preambles with sizeequal to the number n₁, and generating the second group of one or morepreambles with size equal to a difference between the number n_(TOTAL)and the number n₁. The first group of preambles may be reserved forPhysical Random Access Channel (PRACH) transmissions bynon-link-budget-limited UE devices (and/or legacy devices). In contrast,link-budget-limited UE devices may be configured to perform PRACHtransmissions using the second group of one or more preambles.

After having broadcast the system information, the processing agent mayreceive a PRACH, i.e., a PRACH that has been transmitted by a UE device.The processing agent may determine whether the PRACH includes a preamblefrom the first group or a preamble from the second group.

In response to determining that the PRACH includes a preamble from thesecond group, the processing agent may invoke one or more communicationenhancement mechanisms for transmission to and/or reception from the UEdevice that transmitted the PRACH. (For example, the processing agentmay boost the power of the random access response message, and/or, boostthe power of downlink traffic transmissions to the UE device.) On theother hand, if the processing agent determines that the PRACH includes apreamble from the first group, the processing agent may not invoke theone or more communication enhancement mechanisms.

Improving Reception of Random Access MSG2 for Link-Budget-LimitedDevices

According to the existing LTE specifications, for each random accessattempt, the UE may randomly select a preamble from a group ofpreambles, and transmit the selected preamble in msg1 (i.e., the PRACHmessage). The PRACH message may be transmitted to the network (NW) viathe base station. The UE then waits for msg2, i.e., the so-called randomaccess response message from the base station. Msg2 may be received inthe PDCCH and PDSCH of a downlink subframe within a specific time windowthat is related to the time of transmitting msg1. If msg2 is notreceived in the time window, the UE backs off a certain amount of time,and randomly selects another preamble from the group, and makes anotherrandom access attempt by transmitting another PRACH message (includingthe newly selected preamble).

Link-budget-limited UE devices have an increased likelihood of missing(e.g., not successfully decoding) msg2 as compared to UE devices thatare not link budget limited. If the eNodeB knows that a UE device islink budget limited, the eNodeB can boost the power of msg2 to enableincreased likelihood of successful decode of msg2 by the UE device.However, in some situations, the base station may not be able todetermine from msg1 whether the UE is link budget limited.

In some embodiments, a link-budget limited (LBL) device may operate asfollows.

1) The LBL device may randomly select a preamble from the group ofpreambles as defined by 3GPP specifications, and transmit the preamblein msg1 of the random access procedure. This transmission may bereferred to as the first preamble transmission. If msg2 is receivedwithin the expected time window after a transmission of msg1, the LBLdevice may transmit msg3 of the random access procedure.

2) If the LBL device fails to receive msg2 within the expected timewindow after a transmission of msg1, another random access attempt maybe initiated. In particular, the LBL device may: back off a certainamount of time; and perform another transmission of msg1 (using the samepreamble as the first preamble transmission, or alternatively, a nextpreamble in a sequence of preambles). Backing off means waiting anamount of time before transmitting. The backoff amount may, e.g., bedefined relative to the RA (Random Access) window after transmission ofMSG1, and may be fixed or random. For example, the backoff amount may bea fixed value for all preamble transmissions after the firsttransmission, and/or, for all LBL devices in the cell.

Thus, the LBL device may repeatedly transmit msg1 until msg2 issuccessfully received. The repeated transmissions of msg1 may use thesame preamble. Alternatively, the repeated transmissions of msg1 may usea sequence of preambles whose pattern is known to the eNodeB. Successivetransmissions of msg1 may use successive preambles from the sequence.(While the first preamble of the sequence may be randomly selected, therelationship between successive preambles of the sequence may be knownto the eNodeB.)

In some embodiments, the eNodeB may perform the following operations.

1) The eNodeB may count the number of times that: msg1 has been receivedwith the same preamble, and the corresponding msg2 procedure has failed.In other words, for a given preamble, the eNodeB may count the number oftimes where the eNodeB:

-   -   receives msg1 and the msg1 contains the given preamble; and    -   sends msg2, but no msg3 is received from the UE.

2) If the number of times reaches (or alternatively, exceeds) athreshold value N (e.g., N=2, 3, 4, 5 or 6), then the eNodeB can powerboost the transmission of msg2 in response to one or more followinginstances of msg1 that contain the given preamble, until msg3 isreceived from the UE device. For example, the PDCCH and/or the PDSCH ofmsg2 may be power boosted.

The “same preamble” referred to above assumes the case where theLBL-type UE uses the same preamble (as the first preamble transmission)for msg1 retransmissions. Alternatively, if the LBL-type UE uses apreamble sequence for msg1 transmissions, the eNodeB may follow the samepreamble sequence. Thus, the eNodeB may count the number of times wherethe eNodeB:

-   -   receives msg1 and the msg1 contains a preamble consistent with        the preamble sequence; and    -   sends msg2, but no msg3 is received from the UE.

In some embodiments, an LBL device may be configured to add a specialMAC control element (CE) as an additional MAC PDU header in msg3, toindicate to the NW that it is link budget limited. When the base stationreceives msg3, the base station may determine whether or not the UE thattransmitted the msg3 is link budget limited by determining if thespecial MAC CE is present in the msg3.

In one set of embodiments, an LBL device may transmit a first PRACHmessage using a first preamble that is randomly selected from anavailable set of preambles. If the random access attempt correspondingto a previous PRACH message (e.g., the first PRACH message) is notsuccessfully completed, the LBL device may transmit an additional PRACHmessage. Thus, the LBL device may transmit a succession of PRACHmessages until random access is successfully completed. The successionof PRACH messages may have preambles conforming to a sequence ofpreamble index offsets. (The LBL device may randomly select the sequenceof preamble index offsets from a predetermined set of offset sequences.The predetermined set of offset sequences is known to the eNodeB.) EachPRACH message after the first PRACH message may include a respectivepreamble determined by:

-   -   (a) a respective index offset from the selected sequence of        index offsets; and    -   (b) the index I₀ of the first preamble.        For example, the k^(th) PRACH message after the first PRACH        message may include the preamble identified by index        I₀+offset(k), where offset(k) is the k^(th) offset in the        selected sequence of index offsets.

In some embodiments, the random selection from the predetermined set maybe based on the cell ID of the eNodeB so that LBL devices in differentcells will select different index offset sequences from thepredetermined set.

The LBL devices may use index offset sequences selected from thepredetermined set of offset sequences. In contrast, when a non-LBLdevice (and/or a legacy device) experiences failure on any given randomaccess attempt (e.g., by virtue of missing msg2), it may randomly selectanother preamble from the available set of preambles, and transmitanother PRACH message based on the randomly selected preamble. Thus,successive PRACH transmissions from a non-LBL device will generally notbe consistent with any of the sequences of the predetermined set. (Theprobability that a non-LBL device will randomly select a sequence ofpreambles consistent with any of the sequences of the predetermined setis very low.)

The eNodeB may count the number of failed RACH attempts whose preamblesare consistent with a given index offset sequence. As the count grows,the likelihood that the RACH attempts are associated with an LBL deviceincreases. When the eNodeB receives a current PRACH message that isconsistent with the given index offset sequence, the eNodeB maydetermine if the count has reached (or alternatively, is greater than) athreshold value N. If so, the eNodeB may transmit the random accessresponse message (i.e., random access msg2) with increased powerrelative to the power (or powers) used for the first N transmissions ofmsg2. In some embodiments, the first N transmissions may be transmittedwith a given power P₀, and any transmission of msg2 after the first Ntransmissions may be transmitted with power (or powers) greater than P₀.In one embodiment, successive transmissions of msg2 after the Nthtransmission are transmitted with increasingly greater power.

In one set of embodiments, a method 1600 for operating a user equipment(UE) device may be performed as illustrated in FIG. 16A. (Method 1600may also include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-15 and described below inconnection with FIG. 17.) The method 1600 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent. The processingagent may be realized by one or more processors executing programinstructions, by one or more programmable hardware elements, by one ormore dedicated hardware devices such as ASICs, or by any combination ofthe foregoing.

While method 1600 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1610, the processing agent may perform one or more iterations of aset of operations until a termination condition is achieved. The set ofoperations may include the operations 1615 through 1620 described below.

At 1615, the processing agent may generate a preamble for a PRACHmessage.

At 1620, the processing agent may transmit the PRACH message to a basestation, wherein the PRACH message includes the preamble. Thetermination condition may be the condition that the UE devicesuccessfully receives a random access response (RAR) message that isresponsive to the PRACH message.

The one or more preambles in the one or more respective transmissions ofthe PRACH message may be generated based on:

-   -   a sequence of preamble index offsets, where the sequence has        been configured (or reserved) for use by link-budget-limited UE        devices; and    -   a first index of a first preamble generated for a first of the        one or more transmissions of the PRACH message.

In some embodiments, the method 1600 may also include selecting thesequence of preamble index offsets from a predetermined set of preambleindex offset sequences, wherein the predetermined set has beenconfigured (or reserved) for use by link-budget-limited UE devices.

In some embodiments, the action of selecting from the predetermined setis a random selection based on a cell ID of the base station.

In some embodiments, the UE device is link budget limited. A UE devicethat is link budget limited may require more than one iteration of theset of operations to attain the termination condition.

In some embodiments, for each iteration after a first of the iterations,the set of operations also includes backing off by a fixed amount oftime prior to a next one of said transmissions of the PRACH message.

In one set of embodiments, a method for operating a base station may beperformed as described below. (The method may also include any subset ofthe features, elements and embodiments described above in connectionwith FIGS. 1-16 and described below in connection with FIGS. 16B and17.) The method may be performed by a base station to facilitate arandom access procedure for a link-budget-limited UE device. The methodmay be implemented by a processing agent of the base station. Theprocessing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements suchas FPGAs, by one or more dedicated hardware devices such as ASICs, or byany combination of the foregoing. The processing agent may be configuredto transmit wireless signals via a transmitter of the base station andreceive wireless signals via a receiver of the base station, e.g., asvariously described above.

While the method is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

The processing agent may receive a current PRACH message, e.g., from aUE device in the cell. A UE device of link-budget-limited type may beconfigured to repeatedly transmit a PRACH message using preamblesconforming to a known pattern, until the random access procedure issuccessfully completed. In contrast, UE devices of non-LBL type (and/orlegacy devices) may be configured to randomly select a preamble for eachtransmission of a PRACH message.

The processing agent may increment a failure count in response to adetermination that the current PRACH message results in random accessfailure and that the preamble in the current PRACH message and one ormore previous PRACH message preambles are consistent with a pattern ofpreambles reserved for UE devices of LBL type.

In response to determining that the failure count exceeds (oralternatively, reaches) a threshold value, the processing agent mayinvoke one or more communication enhancement mechanisms for transmissionto and/or reception from the UE device that transmitted the PRACH. (Forexample, the processing agent may boost the power of the random accessresponse message, and/or, boost the power of downlink traffictransmissions to the UE device.) On the other hand, if the processingagent determines that the failure count is less than or equal to (oralternatively, less than) the threshold value, the processing agent maynot invoke the one or more communication enhancement mechanisms.

In one set of embodiments, a method 1650 for operating a user equipment(UE) device may be performed as illustrated in FIG. 16B. (Method 1650may also include any subset of the features, elements and embodimentsdescribed above in connection with FIGS. 1-16 and described below inconnection with FIG. 17.) The method 1650 may be performed by alink-budget-limited UE device to facilitate a random access procedure.The method may be implemented by a processing agent of the UE device.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1650 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1660, in response to a determination that a random access response(RAR) message has not been received after having transmitted a previousPRACH message, the processing agent may perform a set of operationsincluding operations 1665 and 1670 described below. (The processingagent may monitor an expected time window for the RAR message, i.e., anexpected time window after the previous PRACH transmission.)

At 1665, the processing agent may generate a preamble based at least inpart on a current offset in a sequence of preamble index offsets, e.g.,as variously described above. The sequence of preamble index offsets mayhave been configured (or, may be dedicated) for use bylink-budget-limited UE devices.

At 1670, the processing agent may transmit a current PRACH messageincluding the generated preamble.

In some embodiments, the above-described preamble may be generated basedon the current offset and an initial index, e.g., as variously describedabove. The initial index may be an index of an initial preamble used inan initial transmission of the PRACH message.

In some embodiments, the method 1650 may also include selecting thesequence of preamble index offsets from a predetermined set of preambleindex offset sequences, where the predetermined set has been configured(or reserved) for use by link-budget-limited UE devices.

In some embodiments, the action of selecting from the predetermined setis a random selection based on a cell ID of the base station.

In some embodiments, the sequence of preamble index offsets may be analternating sequence, i.e., alternating between two distinct offsetvalues.

In some embodiments, the sequence of preamble index offsets may be acyclic sequence, i.e., cycling through n_(CYC) offset values, whereinn_(CYC) is greater than or equal to two.

In some embodiments, the sequence of preamble index offsets may be asequence of nonzero values, or a sequence of positive values, or asequence that includes two or more non zero values as well as one ormore zero values.

In some embodiments, the sequence of preamble index offsets may be asequence of zero values.

In some embodiments, the UE device is link budget limited.

In some embodiments, the set of operations also includes backing off bya fixed amount of time prior to said transmission of the current PRACHmessage.

In one set of embodiments, a method 1700 for operating a base stationdevice may be performed as illustrated in FIG. 17. (Method 1700 may alsoinclude any subset of the features, elements and embodiments describedabove in connection with FIGS. 1-16B.) The method 1700 may be performedin order to facilitate random access by a link-budget-limited UE device.The method may be implemented by a processing agent of the base station.The processing agent may be realized by one or more processors executingprogram instructions, by one or more programmable hardware elements, byone or more dedicated hardware devices such as ASICs, or by anycombination of the foregoing.

While method 1700 is described below in terms of a number of steps, itshould be understood that in various embodiments: one or more of thesteps may be omitted; two or more of the steps may be performed at leastpartially in parallel; one or more steps may be added, as desired; andthe steps may be performed in different orders than that described.

At 1710, the processing agent may receive a current PRACH message afterhaving received a plurality of previous PRACH messages. The previousPRACH messages:

-   -   (a) have respective preambles agreeing with a sequence of        preamble index offsets, wherein the sequence of preamble index        offsets has been configured (or, is dedicated) for use by        link-budget-limited user equipment (UE) devices and    -   (b) have resulted in random access failure.        A memory of the base station may store a count of the previous        PRACH messages.

At 1715, in response to receiving the current PRACH message, theprocessing agent may transmit a random access response (RAR) message. Apower of the transmission of the RAR message may be less than or equalto a first power level if a current value of the count is less than orequal to a threshold value N, where N is an integer greater than one.Alternatively, the power of said transmission of the RAR message may begreater than the first power level if the current value of the count isgreater than the threshold value N.

In some embodiments, the method 1700 may also include incrementing thecount in response to determining that:

-   -   (1) a preamble of the current PRACH message agrees with an        expected preamble based on a preamble index of a first of the        previous PRACH messages and a next preamble index offset of the        sequence of preamble index offsets; and    -   (2) a third random access message, responsive to the RAR        message, is not received by the base station.

In some embodiments, the predetermined sequence of preamble indexoffsets is a sequence of zero values.

In some embodiments, a first of the link-budget-limited UE devices isconfigured to randomly select the sequence of preamble index offsetsfrom a predetermined set of preamble index offset sequences, and togenerate preambles for successive random access attempts using theselected sequence of preamble index offsets.

Embodiments of the present disclosure may be realized in any of variousforms. For example some embodiments may be realized as acomputer-implemented method, as a computer-readable memory medium, or asa computer system. Other embodiments may be realized using one or morecustom-designed hardware devices such as ASICs. Still other embodimentsmay be realized using one or more programmable hardware elements such asFPGAs.

In some embodiments, a non-transitory computer-readable memory mediummay be configured so that it stores program instructions and/or data,where the program instructions, if executed by a computer system, causethe computer system to perform a method, e.g., any of a methodembodiments described herein, or, any combination of the methodembodiments described herein, or, any subset of any of the methodembodiments described herein, or, any combination of such subsets.

In some embodiments, a device (e.g., UE 106) may be configured toinclude a processor (or a set of processors) and a memory medium, wherethe memory medium stores program instructions, where the processor isconfigured to read and execute the program instructions from the memorymedium, where the program instructions are executable to implement amethod, e.g., any of the various method embodiments described herein(or, any combination of the method embodiments described herein, or, anysubset of any of the method embodiments described herein, or, anycombination of such subsets). The device may be realized in any ofvarious forms.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

What is claimed is:
 1. A method for operating a user equipment device,the method comprising: receiving a logical root sequence number that hasbeen broadcast by a base station; generating a set of preambles based ondata including the logical root sequence number, wherein said generatingincludes determining a first physical root sequence number according toa conventional mapping of logical root sequence numbers to physical rootsequence numbers, wherein the set of preambles is generated by:generating a sequence of physical root sequence numbers based on thereceived logical root sequence number; and applying cyclic shifts toroot sequences that correspond to the physical root sequence numbers ofsaid sequence, wherein the set of preambles includes: a first subset ofpreambles for transmission of a Physical Random Access Channel (PRACH)by non-link-budget-limited UE devices; and a second subset of preamblesfor transmission of the PRACH by link-budget-limited UE devices, whereinthe first subset and the second subset are disjoint subsets; in responseto a determination that the UE device has been classified as being linkbudget limited, transmitting the PRACH to the base station using one ofthe preambles from the second sub set.
 2. The method of claim 1, whereinthe base station is also configured to broadcast the logical rootsequence number and a parameter Ncs as part of a system informationbroadcast, wherein said data also includes the parameter Ncs.
 3. Themethod of claim 1, wherein the first subset of preambles corresponds toa set of preambles defined by the LTE standard.
 4. The method of claim1, further comprising: in response to a determination that the UE devicehas been classified as not being link budget limited, transmitting thePRACH to the base station using one of the preambles from the first subset.
 5. The method of claim 1, wherein the UE randomly selects saidpreamble from the second subset of preambles.
 6. A method for operatinga user equipment (UE) device, the method comprising: receiving systeminformation that has been broadcast by a base station, wherein thesystem information includes: a logical root sequence number; a totalnumber of preambles included in a set of preambles, wherein the set ofpreambles includes a first group of preambles and a second group of oneor more preambles, wherein the first group and the second group aredisjoint; a first number of preambles in the first group, wherein thefirst number is positive but smaller than the total number; generatingthe set of preambles based on data including the logical root sequencenumber, wherein said generating includes generating the first group ofpreambles with size equal to the first number, and generating the secondgroup of one or more preambles with size equal to a difference betweenthe total number and the first number, wherein the first group ofpreambles is reserved for Physical Random Access Channel (PRACH)transmissions by non-link-budget-limited UE devices, wherein the secondgroup of one or more preambles is used for PRACH transmissions bylink-budget-limited UE devices; in response to a determination that theUE device has been classified as being link budget limited, transmittingthe PRACH to the base station using one of the one or more preamblesfrom the second group.
 7. The method of claim 6, wherein the systeminformation includes a Random Access Channel (RACH) ConfigCommon messageconforming to the LTE standard, wherein the RACH-ConfigCommon messageincludes the total number, the first number and a message size, whereinthe base station is configured to set the message size equal to valuesufficiently large to decrease a number of non-link-budget-limited UEdevices that use preambles from the second group.
 8. The method of claim6, wherein the system information includes a RACH-ConfigCommon messageconforming to the LTE standard, wherein the RACH-ConfigCommon messageincludes the total number, the first number and a power offset, whereinthe base station is configured to set the power offset to a valuesufficiently large to increase a number of link-budget-limited UEdevices that use preambles from the second group.
 9. The method of claim6, wherein the system information also includes a parameter Ncs, whereinsaid data includes the parameter Ncs.
 10. The method of claim 6, whereinthe system information includes a RACH-ConfigCommon message conformingto the LTE standard, wherein the RACH-ConfigCommon message includes thetotal number, the first number, an indication of a power offset and anindication of a message size threshold, wherein the method furthercomprises: in response to a determination that the UE device has beenclassified as not being link budget limited, performing operationsincluding: determining if (a) a path loss measured by the UE device isless than a path loss threshold determined in part from the power offsetand (b) a size of an uplink message to be transmitted by the UE deviceis greater than the message size threshold; in response to determiningthat (a) and (b) are true, selecting a preamble from the first group;and transmitting a PRACH including the selected preamble.
 11. The methodof claim 6, wherein a medium access control header of a message from theuser equipment device during a random access process indicates that theuser equipment is link budget limited.
 12. A method for operating a userequipment (UE) device to facilitate random access to a wirelesscommunication network, the method comprising: in response to adetermination that a random access response (RAR) message has not beenreceived after having transmitted a previous physical random accesschannel (PRACH) message, performing a set of operations including:generating a preamble based at least in part on a current offset in thesequence of preamble index offsets, wherein the sequence has beenconfigured for use by link-budget-limited UE devices, wherein thepreamble is generated based on the current offset and an initial index,wherein the initial index is an index of an initial preamble used in aninitial transmission of the PRACH message; and transmitting a currentPRACH message including the generated preamble.
 13. The method of claim12, further comprising: selecting the sequence of preamble index offsetsfrom a predetermined set of preamble index offset sequences, wherein thepredetermined set has been configured for use by link-budget-limited UEdevices.
 14. The method of claim 13, wherein said selection from thepredetermined set is a random selection based on a cell ID of the basestation.
 15. The method of claim 12, wherein the sequence of preambleindex offsets is a sequence of zero values.
 16. The method of claim 12,wherein the set of operations also includes backing off by a fixedamount of time prior to said transmission of the current PRACH message.17. A method for operating a base station, the method comprising:receiving a current physical random access channel (PRACH) message afterhaving received a plurality previous PRACH messages, wherein theprevious PRACH messages (a) have respective preambles that agree with asequence of preamble index offsets that has been configured for use bylink budget limited user equipment (UE) devices and (b) have resulted inrandom access failure, wherein memory of the base station stores a countof the previous PRACH messages; in response to receiving the currentPRACH message, transmitting a random access response (RAR) message,wherein a power of said transmission of the RAR message is less than orequal to a first power level if a current value of the count is lessthan or equal to a threshold value N, wherein the power of saidtransmission of the RAR message is greater than the first power level ifthe current value of the count is greater than the threshold value N.18. The method of claim 17, further comprising: incrementing the countin response to determining that: a preamble of the current PRACH messageagrees with an expected preamble based on a preamble index of a first ofthe previous PRACH messages and a next preamble index offset of thesequence of preamble index offsets; and a third random access message,responsive to the RAR message, is not received by the base station. 19.The method of claim 17, wherein the predetermined sequence of preambleindex offsets is a sequence of zero values.
 20. The method of claim 17,wherein a first of the link-budget-limited UE devices is configured torandomly select the sequence of preamble index offsets from apredetermined set of preamble index offset sequences, and to generatepreambles for successive random access attempts using the selectedsequence of preamble index offsets.